Reduction and Slicing of Hierarchical State Machines Mats Heimdahl et al. University of Minnesota Presented by Tom McMullen For CISC836 1

Outline Primer Problem Space Paper Overview Application (Case Study) Limitations Critique Discussion 2

Primer Presented at 1997 Proceedings 6 th European Software Engineering Conference Cited by 65 (source: Google Scholar) 3

Problem Space Formal Specification Languages – Difficult to understand and use – Not well understood by application experts 4

Problem Space State Machine Representation – Better, but… Inevitable Complexity for large systems 5

Problem Space 6

What this paper proposes… Address complexity of HSMs In Order To: – Present information in digestible chunks Method: – Step 1: Simplify based on scenario – Step 2: Slice for desired values 7

A Quick Refresher… Hierarchical State Machines (HSM) 8

A Quick Refresher… Slicing 9

A quick refresher… Program Slicing 10

Application Applied to HSMs – RSML (Requirements State Machine Language) Specification of safety-critical systems 11

RSML 12

Testbed Specification TCAS II – Traffic alert and Collision Avoidance System 13

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Step 1: Reduce Produce simplified RSML model Interpretation based on scenario – Domain restriction of next-state relation – How do we classify an intruder who has stopped reporting altitude? 15

Step 1: Reduce Eliminate infeasible columns for scenario Reduction Algorithm limited to enumerated vars 16

Step 2: Slice Slicing Algorithm – Based on marking of Abstract Syntax Tree – RSML parser part of earlier research Data Flow Slices (if a transition can be taken) – Data Dependency of Guarding Transition Control Flow Slices (when a transition is taken) – Generation of a trigger event 17

Step 3: Profit?... Iterative slices are then combined Attempt to answer our questions Reduced complexity Increased Understandability 18

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Case Study Applied to most complex part of TCAS II RSML Model Subjective notion of complexity / understandability Metrics: – # of transitions – Perceived Table Size – Effective Table Size 20

Case Study Results – Promising Significant reduction in table size – But.. Reduction in # of transitions not as expected 21

Critique Case Study details lacking – Which TCAS model? What Questions/Scenario? – 1998 paper has a more detailed case study/empirical data Too much emphasis on RSML – Small application – More detail on slicing HSM in general Reduction and Slicing algorithms not presented – Data flow / control flow slices not explained in depth 22

Discussion 23